For quite some time, there has been a desire to provide vacuum cleaners with a measuring and indicating device that allows the user to assess how the level of cleanliness of the surface to be treated varies during the vacuum cleaning process. In fact, it would be of great help to the operator if he or she could see from an indicating device that, after a certain vacuum cleaning time, there remains hardly any dust-laden air being conveyed through the flow-conveying system of the vacuum cleaner, so that the vacuum cleaning process is not unnecessarily prolonged. To remedy this, a number of proposals have been made in the patent literature, but these proposals have not yet led to any implementation that would be relevant in practice.
For example, it is known from European publication EP 0 231 419 A1 to provide an indicating and control device in the form of an attachment or accessory for a vacuum cleaner, said indicating and control device being installable in the flow-conveying system of the vacuum cleaner. This indicating and control device should be activatable by a low-pressure switch and operate with optical dust-detecting means. It was also proposed to mount this unit on the vacuum cleaner or its parts (suction brush, suction wand, handle, suction hose) at a position adjacent to the suction region of the vacuum cleaner. That prior publication does not include any specific information on how and where to install the sensor which is at the heart of such a system. There is no known practical implementation based on this design approach.
A practical embodiment in which the dust-flow sensing and indicating device is mounted in a suction nozzle is described in DE 93 11 014 U1 and DE 202 07 071 U1, respectively. Both design approaches have in common that the sensing and indicating device is located in the bottom part of the suction nozzle.
In DE 93 11 014 U1, a removable sub-housing is provided above the bottom part of the suction nozzle to accommodate the sensor and the indicating elements. The sensor is formed by optical dust-detecting means provided in the inlet duct or in the inlet region of the bottom part of the suction nozzle. The floor nozzle depicted in that prior publication illustrates the drawbacks of this embodiment particularly well. Since the additional elements (sensor, battery-operated control device, and indicating devices) are accommodated directly in the bottom part of the suction nozzle, the size of the bottom part is inevitably increased. Therefore, it is no longer possible to move such a suction nozzle into the narrow free spaces under furniture. Moreover, in cases where the bottom part of the suction nozzle does disappear under a piece of furniture, the indicating device is no longer visible. Furthermore, the bottom part of the suction nozzle, in particular, is inevitably exposed to strong shocks. This may easily lead to failures of the control electronics located in the bottom part and of the sometimes fragile sensor elements.
These are certainly the crucial reasons why, up to the present time, no suction nozzle having an indicating and sensing device mounted directly in the bottom part of the suction nozzle could be established in the market to an appreciable extent.
All design approaches described above are based on providing a sensor in the form of an optical dust-detecting means which, for system-inherent reasons alone, has certain drawbacks. It is not very easy to provide an optical measurement section in the confined mounting space in the flow channels of the air-conveying system of a vacuum cleaner. Moreover, an optical measuring system is not sufficiently reliable and accurate for certain purposes. A particular disadvantage of optical systems is that, in addition to the actual dust, they also respond to carpet fibers resulting from wear and tear of a carpet, but do not allow any conclusions about the level of cleanliness.
In contrast, a design approach described in EP 0 759 157 B1 is particularly advantageous in that it uses a piezoelectric sensor device which allows even minute dust particles to be reliably detected without the signal being affected so much, for example, by carpet fibers. The embodiment described in that patent proposes for the piezoelectric sensor to be mounted in the inlet tube or in an inlet connector of the vacuum cleaner. When the dust particles present in the air-flow path hit the piezoelectric sensor they transfer part of their energy to the sensor, the piezoelectric sensor converting the kinetic energy into a corresponding electrical signal (piezoelectric voltage). That prior publication does indeed describe the physical sensor system and the electronic control system in detail, but it does not provide any details on where and how to install the individual elements-sensor, control electronics, indicating device and, possibly, a battery provided for power supply.